1. Field of the Invention
This invention relates to a diaphragm mechanism of a photographic lens.
2. Description of the Prior Art
The diaphragm mechanism of the conventional photographic lens has a plurality of diaphragm blades, a fixing member for supporting them, a diaphragm blade driving plate for driving the diaphragm blades, and a stop-down lever integrally attached with the diaphragm blade driving plate. A pin projected on one side of each diaphragm blade is rotatably supported by the fixing member and a pin projected on the other side of each diaphragm blade is fitted in a straight groove formed in the diaphragm blade driving plate. With the movement of the stop-down lever, the diaphragm blade driving plate is rotated to displace the diaphragm blades and control the aperture diameter.
In this case, for example, when the aperture is stopped down from the fully open aperture diameter to the minimum aperture diameter, the aperture diameter is decreased by 1/.sqroot.2 times each time the number of aperture states is varied by one stage (for example, from Fl.4 to F2). In the converse case, as the aperture approaches the fully open aperture diameter, the aperture diameter is increased by .sqroot.2 times for a variation of one stage. That is, the aperture diameter is varied in the manner of a geometric progression. In accordance with this, the stroke of the stop-down lever is also varied in the manner of a geometric progression. In the stroke characteristic of the stop-down lever of FIG. 1 wherein the abscissa represents the stroke of the stop-down lever going from the fully open aperture toward the minimum aperture and the ordinate represents the number of aperture stages, the conventional diaphragm mechanism is represented by a curve (A). In the conventional construction, when the aperture control is effected by the stop-down lever, the more the aperture is stopped down, the smaller is the stroke of the stop-down lever required for the variation of the same number of aperture stages and therefore, the smaller is the aperture opening, the worse is the control accuracy of the aperture diameter.
Now, a diaphragm mechanism is known in which the diaphragm blade driving plate is provided with a cam to vary the stroke of the stop-down lever in the manner of an arithmetic progression for a variation of one aperture stage as indicated by a line (B) in FIG. 1. With this construction, however, the amount of rotation of the diaphragm blades is great particularly in the vicinity of the fully open aperture and therefore, in the vicinity of the first stage, namely, in the vicinity of the fully open aperture diameter, the efficiency of the cam becomes worse due to the friction loss of the cam surface and the rising speed of the stop-down becomes slower to aggravate the follow-up characteristic of the aperture with respect to the movement of the stop-down lever or to increase the time required to stop down the aperture to a required aperture value. Accordingly, if an objective lens provided with such a diaphragm mechanism is used with a camera in which the aperture diameter is automatically controlled in accordance with the brightness of an object, there is a disadvantage that the time required for the aperture control is increased.